The following invention relates to an electric motor construction employing a fluid bearing, and in particular relates to a hydrodynamic fluid bearing in which asymmetric journal grooves create a pressure gradient in the bearing fluid that directs pressure toward a dead end thrust plate cavity.
Because of run out, vibration, and other problems associated with ball bearings in small electric motors, such as the type used for computer disk drives, some motors now make use of fluid hydrodynamic bearings. Bearings of this type are shown in the U.S. patents to Titcomb U.S. Pat. No. 5,112,142, Shinohara U.S. Pat. No. 4,445,793 and Anderson U.S. Pat. No. 4,726,693.
Hydrodynamic bearings provide the advantage that there are no moving surfaces in direct contact with each other. Instead, fluid of a predetermined viscosity used for the bearing is placed in the gap between the motor's shaft and a sleeve formed in the housing. The shaft and the housing sleeve rotate with respect to each other. The shaft typically includes at least one axial load bearing thrust plate. A clearance space or gap is present in the axial direction between the thrust plate and the housing, as well as in the radial direction between the sleeve and the shaft and the outer ends of the thrust plate. Typically the ends of the thrust plate and/or shaft are exposed to the outside atmosphere and a capillary seal is used to retain the bearing fluid.
It is important that with this construction bearing fluids remain free of cavitation. Cavitation can occur when the pressure in the lubricant falls to the vapor pressure of the gasses in the lubricant, which is less than the partial pressure of the dissolved gases. In order to maintain pressure in the fluid, designers typically include equalization ports that allow circulation of the bearing fluid from one gap region to another gap region. In Titcomb U.S. Pat. No. 5,112,142 a plurality of bearing pressure equalization ports connect the clearance spaces for the thrust plates and the shaft to ensure proper equalization of fluid pressures within the bearing. Anderson et al. U.S. Pat. No. 4,726,693 uses a groove pattern journaled into the shaft together with equalization ports to recirculate oil from an oil reservoir to a journal zone. One problem with the designs described above is that all are difficult to machine and to manufacture.
Other devices such as that shown in Shinohara U.S. Pat. No. 4,445,793 include multiple fluid bearing zones between various portions of the shaft, thrust plate and sleeve. This creates opportunities for the bearings to leak because tolerances for these bearing zones are difficult to control in the manufacture of the motors. They also require a number of ambient air ports which are costly to manufacture.